In the production of fiber lightguides which are used, for example, in optical communications systems, a layered preform to be drawn through a high temperature furnace and having an outside diameter of about 12 mm and a length of about 70 cm is suspended with its axis oriented vertically. The furnace may be one such as that shown, for example, in U.S. Pat. No. 4,030,901, which issued on June 21, 1977 in the name of P. Kaiser and which includes a chamber heated to a temperature above 2000.degree. C. and typically filled with an inert gas. As the drawing process is continued, the preform is moved downwardly through an opening in an upper end of the furnace to maintain a lower end of the preform within the heated chamber.
Because of the opening in the upper end of the furnace, a seal must be provided to maintain an inert, relatively high purity atmosphere within the furnace. If a high purity inert atmosphere is not maintained, a heating element and other furnace components will oxidize and deposits will form on the preform and fiber lightguide surface. The provision of a seal at the upper end of the furnace is also necessary to prevent air from being drawn into an opening in a lower end of the furnace and moved through the chamber and out the top, especially when a spent preform is removed from the furnace. Although furnaces are available which do not use inert atmospheres, their heating chambers must be sealed off from ambient conditions to provide heat stability, while preventing turbulence, to control the diameter of the drawn fiber.
It is also desirable that the furnace seal be non-contacting with the preform extending therethrough in order to avoid strength reduction in the drawn fiber. The objectionable characteristics of fibers which are drawn from preforms that have been abraded due to surface contact were discussed in a talk entitled "Effect of Drawing Tension and Preform Surface Condition on the Strength of Optical Fibers" which was presented by B. K. Tariyal and F. P. Partus in October, 1977 at the Ceramics Conference in New Bedford, Pennsylvania.
The seal should also be self-centering to accommodate varying preform characteristics such as different diameter preforms, while having the capability of accommodating preforms which are misaligned with a vertical axis of the furnace. Moreover, the preform frequently develops a bow when it is collapsed after it has been layered. The maximum displacement of the bowed configuration, which may be on the order of 6 mm, must be accommodated by the furnace seal.
The prior art includes U.S. Pat. No. 3,245,334 which shows a non-contacting, but non-adjustable, air bearing type seal through which an elongated material is advanced from one chamber into another, and hereinbefore mentioned U.S. Pat. No. 4,030,908 which shows a contacting seal. In U.S. Pat. No. 3,927,544, sealing boxes at opposite ends of a treating chamber have a clearance from a fibrous material, which is large enough to avoid frictional engagement but small enough to effect a seal. Nowhere in the known prior art is there disclosed a seal which is used in cooperation with a furnace to draw fiber lightguides from a preform and which meets all of the hereinbefore-mentioned requirements.